The Sciences

The Mirrors That Made History

How scientists at LIGO in Louisiana calibrate one of the largest, most precise instruments on the planet to hunt for gravitational waves.


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Photo Credits: Ernie Mastroianni/Discover

LIGO staff scientist Joe Betzwieser (left) and physicist Shivaraj Kandhasamy discuss calibration procedures during the engineering run. The engineers direct a low power laser beam that actually moves the LIGO end mirror a small, but precise, amount. Knowing this number will help them gauge the strength of a gravity wave when it passes through.

Photo Credits: Ernie Mastroianni/Discover

Janeen Romie, the LIGO engineering group leader, coordinates the efforts of her colleagues as they calibrate instruments and attempt to lock the laser on the target mirrors. She troubleshoots problems with alignment lasers and the mirrors that direct their paths.

“There are actually 11 kinds of lasers that are part of the detector system,” she said, for such uses as optical levels, fiber welding, thermal compensation and photon calibration.

Photo Credits: LIGO Lab

A LIGO installation specialist checks the alignment of a laser test beam system at the far end of a LIGO arm.

Photo Credits: Caltech/MIT/LIGO Lab

Technicians inspect a coating on one of LIGO's 88-pound mirrors, known as test masses. Each arm of the LIGO detector holds two test masses which reflect the laser light back and forth 280 times along the 2.5 mile beam tube for a total of 700 miles before exiting to the photo detector. 

Photo Credits: Caltech/MIT/LIGO

LIGO’s photo detector sits inside the dark glass enclosure at the center of this seismic isolation platform. The glass is opaque to virtually all light except the infrared laser. When the detector is locked and running, a change in the laser’s light level, matched by a similar change at LIGO’s twin in Hanford, could indicate a gravitational wave.

Photo Credits: Ernie Mastroianni/Discover

The X-arm of the LIGO detector stretches 2.5 miles into the southern Louisiana wilderness. When the detector is locked and running, researchers who drive this road are required to stay under 10 miles per hour to minimize vibrations.

“It takes an eternity,” said group leader Romie. A concrete tunnel protects the beam tube from the elements and occasional stray bullets from hunters.

Photo Credits: Mike Fyffe/LIGO Lab

The level of seismic isolation demanded by the LIGO interferometer is not easy to achieve and requires complex and precise machinery. LIGO Livingston technicians assemble this seismic isolation subsystem.

Photo Credits: Mike Fyffe/LIGO Lab

Inside a stainless steel chamber, LIGO technicians examine the surface of one of the test mass mirrors that will reflect infrared laser light to measure the effect of gravity waves. After installation, all air was vacuumed from this chamber.

Photo Credits: Caltech/MIT/LIGO Lab

Massive stainless steel tubes, vacuum equipment, and seismic isolation gear are prepared for installation at the corner station of the Laser Interferometer Gravitational-wave Observatory (LIGO) detector in Hanford, Washington. The facility is a near twin of the Livingston detector.

To insure the beam’s integrity, the laser travels through sealed stainless steel tubes, 1.2 meters wide, that hold a vacuum to just one trillionth of earth’s atmosphere, eight times less than open space. This vacuum, says LIGO spokesman William Katzman, has been maintained since 1999. It is the largest sustained ultra-high vacuum in the world and is necessary to prevent any air currents from deflecting the laser’s path. 

Photo Credits: LIGO Lab

More than a year after detecting the first confirmed gravitational waves, researchers were busy at the Laser Interferometer Gravitational-wave Observatory (LIGO) in Livingston, La., upgrading the massive instrument. Building on lessons learned during that historic first run, they expect the improved detector will find more gravitational waves during the second observing run, which began Nov. 30.

LIGO detects gravitational waves by splitting a powerful infrared laser beam in two, then sending the beams at right angles through tunnels to mirrors 2.5 miles away. The beams are recombined upon return. A gravitational wave will warp space and briefly change the relative distance between the mirrors and photo detector situated near the LIGO control room. The difference is astonishingly small, just 1/10,000 of a proton’s diameter, but it can be detected if the mirrors are isolated from all external sources of vibration.

Discover photo editor Ernie Mastroianni visited the facility in November as physicists and engineers were calibrating equipment.

Photo Credits: Ernie Mastroianni/Discover

In LIGO’s control room, monitors display seismic activity caused by the steady motion of ocean waves (left) and the nearby logging operation (right). Dark bands indicate breaks in activity, including a lunch hour.

Photo Credits: Ernie Mastroianni/Discover

Heavy logging activity less than a half-mile from LIGO’s main entrance creates vibration that engineers must nullify in order to detect the slight presence of far weaker gravitational waves.

Photo Credits: Ernie Mastroianni/Discover

Astrophysicist Stuart Aston monitors external vibrations on the LIGO test mass mirrors during an engineering run in November 2016. Aston's job is to keep optical components isolated from external vibration, and he was not happy as he scanned the data from LIGO’s control room.

“The multi-stage suspensions provide incredible levels of isolation from ground motion,” said Aston, but it wasn’t happening on this day. Less than a half-mile away, a logging crew was plowing a path through the forest, creating massive ground vibrations and swamping the gear that normally nullifies the noise.

Aston was soon driving the 2.5-mile distance to the detector’s far end to fix the glitch.

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